1. Field of the Invention
The present invention relates to photographic optical systems and photographing apparatuses and, more particularly, to a photographic optical system having an optical member, such as a light transmission member for lens protection or an optical filter, in front of a photographic lens, which is suited to video cameras, digital cameras, still video cameras and film cameras, and a photographing apparatus having the photographic optical system.
2. Description of Related Art
It has been known to provide cameras for photography, video cameras, still video cameras and like photographing apparatuses with a protection glass or optical filter in front of (or on the light entering side) of the photographic lens. In most of these photographing apparatuses, the protection glass or optical filter to be used is usually of the flat parallel form. Meanwhile, of the photographing apparatuses, an apparatus using a CCD or like image sensor as the recording medium has suffered from interreflection, because the image sensor and its cover glass have high refractivity at the surfaces thereof. The reflected light from these surfaces return to the members of the photographic lens and their mountings and therefrom are reflected to enter the image sensor again. In the prior art, therefore, there is a problem that ghost is often recorded. Particularly for the protection glass or optical filter, because it is attached to the very front of the lens system, ghost owing to the interreflection from its surfaces is apt to be formed on the image sensing surface of the image sensor, thus making the problem very serious.
With regard to this problem, in Japanese Laid-Open Patent Application No. Hei 6-160779, as the zoom lens has a protection glass at the very front thereof, each of its surfaces is made to have curvature in such a way as to diffuse the ghost on the image sensing surface. This method of reducing the influence of ghost is applied to sufficiently diffuse ghost in the wide-angle region. For this purpose, the protection glass is given large curvature.
Thus, the use of the optical member, such as a protection glass or optical filter, at the front of the photographic lens leads to a possibility of occurrence of interreflection between the surface of the image sensor and the surfaces of the optical member. To effectively prevent this interreflection from producing flare or ghost due to the incidence of reflected light on the image sensor, it is a good idea to make each surface of the optical member have curvature, as disclosed in Japanese Laid-Open Patent Application No. Hei 6-160779 mentioned above.
However, if the curvature of each surface of the optical member is made too strong, many aberrations increase largely, so that the overall optical performance of the photographic optical system is caused to lower. Conversely, if the curvature of each surface of the optical member is made too weak, the amount of light returning to the image sensor increases, leaving the flare or ghost produced at a high level.